It is already known, as disclosed by EP 1 125 347, that there exist self-adapted filters for fine-tuning laser emissions. That disclosure relates to a laser-type light radiation source including a resonant cavity and an amplification medium placed in the resonant cavity, that source being characterized in that a dynamic photosensitive material is placed in the resonant cavity to form a spectral and/or spatial self-adapting filter.
It is also known, as disclosed in EP 1 327 289, that there exists a continuously wavelength tunable monomode laser source with an external cavity comprising:                a resonant cavity having a reflective plane face, means for extracting a fraction of the light flux, and a retro-reflective dispersive device;        at least one amplifier waveguide placed inside the resonant cavity; and        means for controlling the retro-reflective dispersive device that provide continuous tunability therefore;        that laser source being characterized in that it further comprises a photo-refractive component placed in the cavity, and that is sensitive to the wavelength of the laser source, in which component a Bragg grating is formed.        
It is also known, as disclosed by EP 0 375 021, that there exists a tunable semiconductor diode laser with distributed reflection and a method of manufacturing such a semiconductor diode laser. That publication describes a tunable semiconductor diode laser with distributed reflection that comprises a semiconductor body in which a first radiation guide layer is superposed on a first passive layer in which a strip-shaped resonant cavity is formed between two surfaces extending substantially perpendicularly to the layers, in which resonant cavity there are juxtaposed a first section having a first current supply and an associated active region having a p-n junction that, at sufficiently high current, generates, in the forward direction, coherent electromagnetic radiation, the active region lying within the amplification profile of the radiation guide layer and being delimited laterally by a single layer having sides contiguous to other sections, a second section having a second current supply making it possible to vary the refractive index of a portion of the radiation guide layer that lies within the section, and a third section having a third current supply, the portion of the resonant cavity that lies within the third section having a periodic variation of the refractive index in the longitudinal direction, in which semiconductor diode laser the first radiation guide layer is situated over the entire length of the resonant cavity, the semiconductor diode laser being characterized in that it further comprises means whereby the relative intensity of that fraction of the radiation which is generated in the first section, and which is reflected at the junction between the first and second sections, is made small with respect to the intensity of the radiation returning from the second section to the junction.
The following publications are also known:                K. Lui and M. G. Littman, “Novel geometry for single-mode scanning of tunable lasers”, Opt. Lett. Vol. 6, pp. 117-118, 1981.        F. Favre and D. Le Guen, “82 nm of continuous tunability for an external cavity semiconductor laser”, Electron. Lett. Vol. 27, pp. 183-184, 1991.        L. A. Coldren, “Monolithic Tunable Diode Laser”, IEEE Journal on Selected Topics in Quantum Electron., vol. 6, pp. 988-999, 2000.        N. Huot, J. -M. Jonathan, G. Paulit, P. Georges, A. Brun, G. Roosen, Appl. Phys. B 69, pp. 155, 1999.        S. Maerten, N. Dubreuil, G. Pauliat, G. Roosen, D. Rytz, T. Salva, “Laser diode made single-mode by self-adaptive photorefractive filter”, Optics Communications 208, pp. 183-189, 2002.        S. F. Lyuksyutov and O. I. Yuschuck, “Spectral equalization and autosweeping effects in a pulsed dye laser with an intracavity photorefractive element”, Applied Optics, vol. 31, pp. 1217-1220, 1992.        F. Mogensen, H. Olesen and G. Jacobsen, IEEE J. Quantum Electron. QE-21, pp. 78, 1985;        Petitbon, P. Gallion, G. Debarge, C. Chabran, IEEE J. Quantum Electron. QE-24, pp. 148, 1988.        
There are three categories of controlling the spectra emitted by lasers and, more particularly, the spectra emitted by wavelength tunable lasers. The first category uses frequency filters that are said to be “static” (their frequency responses are fixed) and that are inserted into the cavity of the laser. The second category uses filters that are said to be “dynamic” their frequency responses are not invariable). Finally, the third category covers methods of optically injecting a coherent external beam (coming from another laser) into the cavity of the laser to be controlled.
For each of the categories, we list the technological limits of the methods described in the context of the following need: having N wavelength tunable laser sources (e.g. in the context of optical communications systems). The context is a use in which it is necessary to have a plurality of wavelength tunable laser sources that operate simultaneously and that can be reconfigured (programmed) independently of one another.
A first method of controlling the emission spectrum of a laser includes inserting frequency selective optical filters into the cavity of the laser. A first class of filters exists that are said to be “static”, such as, for example, interference filters, diffraction gratings, Fabry-Perot etalons, etc. Once the filter is inserted into the cavity, it induces fixed loss differences between the modes of the laser, and can lead to laser emission in a single mode: the laser is then said to be “monomode” or “single-mode” (or “monochromatic”). The emission wavelength of the laser is the wavelength for which the filter induces minimized losses. It is possible to vary the wavelength by acting on the filter. When a diffraction grating is used, the emission wavelength is set by the angle of incidence of the beam on the grating. By varying the angle, a wavelength tunable source is obtained that has numerous uses. In the field of optical communications, such laser diode sources, of the extended cavity type, have continuous tunability over a range of 100 nm. Such sources, which are relatively costly, are widely used for characterizing passive and active optical components. They cannot be inserted into transmission links. However, for several years now, compact tunable laser diode sources have existed that are manufactured on the basis of integrating all of the necessary functionality features into the same chip. Such sources are laser diodes of the Distributed Bragg Reflector or “DBR” type, and they are increasingly used in optical transmission networks (to satisfy the needs of reconfiguring optical communications networks).
The limits of such a method are not so much technological as economic. Whenever an application requires a large number of tunable sources, the problem of cost inevitably arises. Unfortunately, such tunable sources are technologically highly advanced, costly to manufacture and to characterize. In addition, for DBR-type diodes, wavelength tuning is performed by acting on three different parameters (which correspond to three currents to be injected into three different sections of the diode). Full characterization of the diode is necessary to determine the triplet of currents to be applied to cause the diode to operate at the desired wavelength. Those operating points are stored in an electronic card. This explains their relatively high utilization cost and management of operation in parallel of N sources that is somewhat complicated (the operating points of all N sources must be characterized and stored in a memory).
Research has been conducted on laser cavities into which “dynamic” filters are inserted. Unlike static filters, their frequency responses are not fixed and adapt to the modes structure of the laser. Such a filter has a dynamic holographic medium in which the modes structure of the laser records a hologram which then acts on the losses of those modes. Those losses are different for the various modes, and they lead, after a competition phenomenon, to a reduction in their number. The hologram adapts continuously to the new modes structure so that, in some cases, it selects only one mode. Inserting a photorefractive crystal (which acts as a dynamic holographic medium) leads to a reduction by a factor of 300 in the width of the spectrum of a Ti laser: sapphire operating under pulsed conditions, and to operation under monomode conditions of an Nd continuous laser: YVO4−. Such lasers oscillate initially, without any holographic medium, under multimode conditions. It should be noted that, in the particular case of the Nd laser: YVO4, the cavity does not include any frequency selective element, other than the hologram that is written in the photorefractive crystal. Identical results have been obtained in an extended-cavity laser diode source. The dynamic nature of the filter enables it to adapt to the operating point of the laser. In the event of a change of operating point of the laser, the filter adapts to the new modes structure and keeps a monomode oscillation. However, without the use of any other static filter, it is impossible, a priori, to predict the wavelength at which the laser will oscillate after the hologram has been written. The only predictable event is that the spectrum of the laser, which is initially multimode, is rendered monomode after the holographic medium is inserted into the cavity.
By combining a static filter and a dynamic filter in the cavity, it is, however, possible to set the operating wavelength of the laser as shown in S. F. Lyuksyutov and O. I. Yuschuk, “Spectral equalization and autosweeping effects in a pulsed dye laser with an intracavity photo-refractive element”, Applied Optics, vol. 31, pp. 1217-1220, 1992. In a first stage, the laser operates in the presence of the grating and of the crystal in which a hologram is recorded. It then oscillates simultaneously at a plurality of wavelengths. Then, by causing the laser to operate with the crystal alone in the cavity, the laser continues to oscillate at the same wavelengths for a short time of a few minutes. After a few minutes, the oscillation wavelength changes.
When the cavity has no frequency selective element other than a dynamic holographic filter, comprising, for example, a photorefractive crystal, the limit of that type of cavity comes from the fact that the wavelength at which the laser oscillates cannot, a priori, be predicted.
Another method is well known, that method being a method of controlling the spectrum of a laser and being constituted by optical injection. It provides a “master” laser whose beam is injected into a “slave” laser. Under certain conditions, related to the power of the beam coming from the master laser and to the mismatch between the oscillation frequency of the master laser and the frequency of one of the resonant modes in the cavity of the slave laser, the slave laser can then oscillate at a wavelength that is similar to or even identical to the wavelength of the master laser.
These properties are kept so long as the injection of the beam coming from the master laser is effective. As soon as the beam disappears, the slave laser resumes its initial operating conditions.
If a wavelength tunable master laser is provided in this case, it is possible to have one or more tunable slave lasers. However, all of the slave lasers operate at the same wavelength (set by the wavelength of the master laser). It is impossible to configure one of the slave lasers independently from the others. All of the slave lasers are dependent on one another.
It would therefore be advantageous to store the operating wavelength (state) of the injected slave laser in a memory after the control (injection) beam coming from the master laser has been interrupted.